The present invention relates to a film forming apparatus used, for example, in the manufacturing process for a semiconductor device.
In the past, in semiconductor devices, a polysilicon film has been used in a wide range of structures such as a gate electrode of a transistor. Particularly with the fineness of the device being in a deep submicron area, as in the case where an embedded electrode of a contact hall is formed, a silicon film as a highly reliable material is indispensable. In view of such a demand, at present, a study on the method for doping an impurity onto the polysilicon film needs to progress further in the future.
Incidentally, known methods for forming a polysilicon film with an impurity, for example, a phosphor (P), so far include a method for hammering phosphor into a polysilicon film by ion implantation and thereafter applying an annealing process, or a method for forming a P.sub.2 O.sub.5 film on the surface of the polysilicon film using a POCl.sub.3 gas and thereafter applying a diffusion process. Further, as a method making use of a HOT-WALL type reduction CVD, an in-situ method (a method for simultaneously carrying out film production and doping) is also known.
However, the method for doping phosphor into the polysilicon film by the ion implantation has a drawback in that crystals in the polysilicon are broken by the shock of the ion implantation. Further, the method using POCl.sub.3 gas has a drawback in that the concentration evenness widthwise of wafers is poor and the step of shaving the P.sub.2 O.sub.5 film is necessary.
On the other hand, the in-situ method is free from those drawbacks as noted above and has the merit in that the phosphor concentration in the film can be controlled by adjusting the flow rate of doping gas. Therefore, the in-situ method is widely carried out, for example, by the vertical type heat treatment apparatus.
In the case where the in-situ method is carried out using the vertical type heat treatment apparatus, when a design is made so that a film forming gas and a phosphine gas are supplied from the bottom of the reaction tube of the heat treatment apparatus to form a gas flow toward the top of the reaction tube, the phosphor is consumed and leaned as the gas flow moves upward and therefore the face-to-face (between wafers) evenness with respect to the phosphor concentration of wafers is deteriorated.
In view of the foregoing, the present inventor has studied the vertical type heat treatment apparatus having the construction shown in FIG. 6. The vertical type heat treatment apparatus shown in FIG. 6 will be briefly explained. Reference numeral 9 designates a reaction tube of a double tube construction. At the lower end of the reaction tube 9 are arranged a main gas inlet pipe 90, and a manifold 94 provided with sub-gas inlet pipes 91, 92 and an exhaust pipe 93 which are different in length from each other. First, a wafer boat 95 having a number of, for example, 100 wafers W mounted thereon is carried into the reaction tube 9 from a lower end opening thereof by a boat elevator 97 to load the wafers W into the reaction tube 9. When the wafers W are loaded, the interior of the reaction tube 9 is heated to a fixed temperature by means of a heater 96. Next, for example, a monosilane gas (SiH.sub.4) gas and a phosphine (Ph.sub.3) gas are diluted with, for example, nitrogen gas and supplied from the main gas inlet gas 90 into the reaction tube 9 while the exhaust pipe 93 vacuums the reaction tube 9 so that the interior of the reaction tube 9 assumes a fixed vacuum degree. At the same time, when the phosphine (Ph.sub.3) gas is diluted with, for example, nitrogen gas and supplied into the reaction tube 9 from the extreme end openings of the sub-gas inlet pipes 91 and 92, a thin film is formed on the wafer surface by the gas phase reaction of the gas to be processed.
According to the aforementioned heat treatment apparatus, since the phosphor for a short portion is compensated for from the sub-gas inlet pipes 91 and 92, the face-to-face evenness of phosphor concentration is enhanced as shown in FIG. 7. Since the phosphine gas flowing out of the extreme end openings of the sub-gas inlet pipes 91 and 92 is partly diffused at the rear side thereof, the bottom portion of the phosphor concentration in the graph shown in FIG. 7 is to be positioned at the lower side (upstream side) rather than the extreme ends of the sub-gas inlet pipes 91 and 92.
Further, as for another technique, there is a technique disclosed in Japanese Patent Laid-Open Publication No. 45529/1995. According to this process, the extreme end of a reaction gas inlet pipe is closed, and one or more nozzles (doping gas supply pipes) different in length from each other having a plurality of gas inject holes arranged in the midst of the reaction gas inlet pipe are provided to enhance the face-to-face evenness of phosphor concentration.
With the recent trend of higher integration of the semiconductor device and more fineness of a pattern thereof, an allowable range of quality of film is considerably narrow. In the heat treatment apparatus shown in FIG. 6, the phosphine gas is diffused out of the extreme end openings of the sub-gas inlet pipes 91 and 92, and the phosphine gas can be compensated for to a comparatively far position downstream though there is some difference depending on the gas flow rate. There is an advantage in that even if the length of the sub-gas inlet pipes 91 and 92 is set considerably roughly, high face-to-face evenness of phosphor concentration is obtained, facilitating the setting of the apparatus.
However, as will be understood from FIG. 7, it is unavoidable in this heat treatment apparatus that, in the phosphor concentration distribution curve, the top and the bottom of the curve are formed in the vicinity of the extreme end of the pipe and upstream somewhat from the extreme end of the pipe, respectively. To cope with this, one proposal is to increase the number of sub-gas inlet pipes. In this proposal, however, the wafer possibly comes in contact with the sub-gas inlet pipe to separate a formed thin film, resulting in the high probability of generation of particles and rendering the maintenance work troublesome, which is not advisable.
Further, the technique disclosed in Japanese Laid-Open Patent Publication No. 45529/1995 previously mentioned is intended to secure the face-to-face evenness of phosphor concentration by phosphine gases blown out of the plurality of holes arranged lengthwise of the nozzles. However, the pressure of the nozzle lowers near the extreme end of the nozzle, posing a problem in that the setting of a diameter of the hole and the arranging of spacing is troublesome. There is a further problem in that, when the setting of the phosphor concentration of wafers varies, the setting of the hole of the nozzle should be changed accordingly.